[Color Computer] [Coco] Bug in FOR NEXT ?

[Frank Pittel]

I always that the loop counter was "undefined" outside the loop. By undefined
I mean that the value is implementation dependant and not guaranteed to be any
specific value.


On Thu, Jun 01, 2006 at 05:31:18PM -0400, Arthur Flexser wrote:
> There is actually a genuine bug in the way CoCo Basic processes FOR/NEXT.  It
> was in fact the subject of an article in Byte in the early 80s because the same
> bug occurs in various other Microsoft Basics as well.  My memory is a bit hazy
> on the details, but here's what I recall;  perhaps someone here can concoct a
> Basic program that illustrates the bug.  I believe that the bug produces a
> spurious ?NF (NEXT without FOR) error.  I did once encounter it in a program I
> wrote, soon after I bought my CoCo and before seeing the Byte article;  I called
> Tandy about it (back in those days they actually had phone support), but the
> techie I spoke with was stumped, not surprisingly.
> 
> The bug occurs when you exit an initial FOR/NEXT loop without completing it
> (which is perfectly legal to do) and then reuse the same loop variable name in a
> later loop.  It is critical that the NEXT statements contain the variable name
> (i.e., NEXT Z, not just NEXT) for the bug to occur.  I think maybe the variable
> that was previously used in an inner loop must be used later in an outer loop,
> or possibly vice versa.  As I said, my memory is pretty vague on this, even
> though I did at one time trace the source of the bug in the ROM code.
> 
> Anyone want to fiddle around with coming up with some code that produces the
> spurious ?NF error?
> 
> Art 
> 
> 
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